Neural Plasticity: Nature's Double-Edged Sword

Excerpted from
Endangered Minds: Why Children Don't Think And What We Can Do About It
By Jane M. Healy, Ph.D.

The large auditorium is hushed as the lights dim and a statistical chart appears on the screen. I reflect momentarily that I have never heard a large group of educators this quiet.

"Now, I'll show you the effects of different environments on our animals' brains." Dr. Marian Diamond wields her laser pointer triumphantly. "We've been working at this for more than thirty years, so I hope you'll forgive me if I skip just a little. " The audience chuckles appreciatively and subsides into rapt attention as Dr. Diamond continues. "Here's a summary of the data comparing brain size and weight of rats reared in the standard cages, those who lived in the 'impoverished' environments, and here"-she pauses dramatically- "are the results with the animals who lived in the enrichment cages. Notice how, with increasing amounts of environmental enrichment, we see brains that are larger and heavier, with increased dendritic branching. That means those nerve cells can communicate better with each other. With the enriched environments we also get more support cells because the nerve cells are getting bigger. Not only that, but the junction between the cells-the synapse-also increases its dimensions. These are highly significant effects of differential experience. It certainly shows how dynamic the nervous system is and how responsive it is to its internal and external surroundings."

This international audience has gathered to hear many speakers describe new concepts for education, but Dr. Diamond is clearly the star attraction. A professor of neuroanatomy at the University of California, Berkeley, she has pioneered studies that have opened scientists' eyes-and minds-about the power of environmental factors in physically altering the dimensions of growing brains. In experiments described in her book Enriching Heredity and elaborated on in the next chapter, rats in an "enriched" environment, actively interested and challenged by frequent new learning experiences, develop larger and heavier brains and also show increased ability to run mazes, the best available test of a rat's intelligence. Moreover, in a series of recent experiments, she has demonstrated for the first time that the effects of personal involvement in new learning appear to be so powerful that rats of any age can develop new brain connections if they intensely pursue new challenges. "Yes," she concludes, with a flourish, "if we work hard enough at it we can even change the very old brain."

She is immediately besieged with questions. Aren't there some basic learning abilities the environment can't change? What about heredity? "Heredity plays a highly important role in the form of these different [behavioral] repertoires," she acknowledges, "but we now have clear evidence that the environment can play a role in shaping brain structure and, in turn, learning behavior. It is the area of the brain that is stimulated that grows."

The auditorium resonates with an undercurrent of response. An elementary school principal seated next to me whispers, "If this applies to human brains, too, think of the implications for teachers-and for parents!"

I am eager to talk with Dr. Diamond, and an hour later, when she has finally been released by a swarm of questioners, I have my chance. This world-renowned scientist turns out to be an approachable and thoughtful person-and it soon becomes evident that she takes her own theories to heart. Our conversation takes place as we stride vigorously through a nearby woods, impelled by the enthusiasm with which she approaches new ideas as well as new physical challenges. She has just returned from her first kayaking trip and is about to embark on a six-week teaching assignment in Africa.

Although Dr. Diamond is obviously convinced that stimulation is good for human as well as for rat brains, I am curious about how confidently we can apply her animal research to children. I explain my questions about the effects of contemporary culture on children's brains. Do neuroanatomists believe that the brains of children, like those of the rats, can be changed by their environments?

"To those of us in the field, there is absolutely no doubt that culture changes brains, and there's no doubt in my mind that children's brains are changing," she replies. "Whatever they're learning, as those nerve cells are getting input, they are sending out dendritic branches. As long as stimuli come in to a certain area, you get more branching; if you lose the stimuli, they stop branching. It is the pattern of the branching that differentiates among us. The cortex is changing all the time-I call it 'the dance of the neurons.' This is true in the brains of cats, dogs, rats, monkeys, or man. "3

Many similar experiments have convinced other scientists of the changeability-they call it plasticity-of brains. Although it is obviously impossible to conduct similar studies on humans, researchers agree both on the validity of principles derived from animal experiments and on the fact that human brains are probably the most plastic of all. Another expert in the field, Dr. Victor H. Denenberg, recently commented, "One would expect even more powerful and more subtle effects with the human, whose brain is vastly more complicated than that of the rat, and who lives in a much more complex social and environmental milieu."

With the reality of brain plasticity well-accepted in scientific circles, it was still a new idea for many of the educators attending Dr. Diamond's presentation.

"I guess it seems obvious, but I somehow never really believed that what I did in the classroom would physically influence the size or shape of my students' brains!" commented one teacher. "It does put being a teacher-or a parent, for that matter-in a whole new light."

Indeed it does. In order to interpret any research responsibly, however, it is necessary to understand it. Although scientists themselves do not claim to have any final answers, this chapter will summarize what is currently known about environments as sculptors of growing minds both before and after birth. Let us start by entangling ourselves briefly in a very old, but fundamental, controversy.

The Adaptable Brain

"Just as the twig is bent, the tree's inclined." Common sense suggests that growing organisms are highly adaptable to external influences, but what seemed so apparent to Alexander Pope has caused psychologists to argue bitterly for years. How much is mental ability shaped by environments and how much is in the hands of heredity? After all, the tree still develops bark, leaves, and a functioning root system no matter how the twig gets bent. Psychologists have tried to resolve this issue with studies comparing identical and fraternal twins. Currently, heredity and environment are each assigned roughly 50 (or 40, or 60)% of the credit. As parents of wiggly little children can understand, however, their physical behavior resists numerical formulas - and so does their mental behavior: learning. So-called "naturenuture" interactions are complex. For example, in a case to be considered in a later chapter, a learning disability that runs in families may result from changes in the child's brain before birth. Cells in the fetal brain get rearranged by chemicals produced because of an inherited response of the mother's own autoimmune system (don't worry, scientists are confused, too)-which the child may also inherit. Would you say this disability is caused by heredity or by the prenatal environment?

In another controversial example, children from lower socioeconomic groups tend to score below average on standard IQ tests. Is this because poor environments depress their intelligence, or because they never learned good test-taking skills, or because, as some believe, families with nonstandard intellectual endowment might get trapped in lower socioeconomic groups? In another chapter, when we consider the results of efforts to alter such children's intelligence, we will see how difficult it is to sort out these factors.

Brain research is now giving these old issues an interesting new dimension by changing the focus from heredity versus environment to heredity plus environment. Until recently, so little has been known about the "brain" that most theorists sidestepped it when trying to explain intelligence (and they produced some mindless theories as a result). Now we acknowledge that the basic genetic architecture for our brains lies at the heart of all learning and even much of our emotional behavior. When these inherited patterns interact with the child's environment, plasticity guarantees an unlimited number of interesting variations. The final pattern is determined by the way each individual uses that unique brain.